Methods suitable for mass production of a glass preform for use in the fabrication of an optical fiber includes a vapor phase axial deposition method (hereinafter referred to as a "VAD" method and an outside vapor phase deposition method (hereinafter referred to as a "OVPD" method). These deposition methods comprise flame hydrolyzing a glass raw material in an oxyhydrogen flame to form glass fine particles of pure quartz (SiO.sub.2) or quartz added containing an additive such as GeO.sub.2 having an average particle size of about 0.1 micrometer, depositing the glass fine particles on a seed member to produce a porous soot preform and sintering the soot preform at a high temperature to obtain a transparent glass preform. According to the VAD method, the glass fine particles are deposited on the rotating seed member in parallel with the rotating axis of the member to continuously form the solid cylindrical soot preform (cf. U.S. Pat. No. 4,135,901). According to the OVPD method, the glass fine particles are deposited on a rotating rod member made of alumina or quartz glass from a direction perpendicular to the rotating axis of the member to form multiple layers of the glass fine particles (cf. U.S. Pat. Nos. 3,711,262, 3,737,292 and 3,737,293). The produced porous soot preform is then heated and sintered in an atmosphere of an inert gas such as helium at a high temperature to make the soot preform transparent to obtain the glass preform. Generally, the porous soot preform contains an additive for adjusting refractive index of the soot preform such as GeO.sub.2. The additive, however, volatilizes from the preform at a high temperature since it is thermally volatile. Therefore, the refractive index adjusted during the production of the soot preform is unacceptably changed due to partial or complete volatilization of the additive during sintering.
To prevent the volatilization of the additive, it is proposed to heat the soot preform at a temperature at least 200.degree. C. higher than the lowest temperature required for making it transparent but not higher than 1,600.degree. C. (cf. Japanese Patent Publication No. 3981/1983).
However, the method disclosed in the Japanese Patent Publication has some drawbacks such that an optical fiber fabricated from the glass preform produced by this method has poor light transmission characteristics, for example, great increase of attenuation of light transmission since the glass preform is produced at a comparatively low temperature. For instance, when a soot preform consisting of glass fine particles of SiO.sub.2 containing 25% by weight of GeO.sub.2 is sintered at 1,350.degree. C. which is 75.degree. C. higher than the lowest sintering temperature to produce a transparent glass preform, an optical fiber fabricated form the glass preform has attenuation of 10 to 20 dB/km at a wavelength of 0.85 micrometer, which is far larger than theoretical critical upper value of about 3 dB/km. This increased attenuation of light transmission is found to be caused by structural defects of the glass preform due to the presence of Ge.sup.2+.
In addition, when the above optical fiber is kept at 200.degree. C., hydrogen, which is present in the fiber, migrated from a covering material of the fiber and/or in air, diffuses to and reacts with the Ge.sup.2+ sites, namely, the structural defects to form GeOH as follows: EQU Ge.sup.2+ +1/2H.sub.2 .fwdarw.GeOH (I)
which leads to increase of residual hydroxyl groups. It is well known that hydrogen easily permeates through a glass material.
Since absorption due to the residual hydroxyl groups has a peak near a wavelength of 1.39 micrometer, it influences light transmission in a wavelength band of 1.30 micrometer which is used in telecommunication and increases the absorption at a wavelength of 1.30 micrometer. Since acceptable increase of attenuation of light transmission in this wavelength band is 1.0 dB/km or less, increase of attenuation by 0.2 dB/km with the passage of time is said to have great adverse effect on communication systems.
The structural defects due to the presence of Ge.sup.2+ in the optical fiber not only deteriorate the light transmission characteristics of the optical fiber but also increase the amount of the residual hydroxyl groups which influence the long-term reliability of the optical fiber. Therefore, it is highly desired to provide a method for producing a glass preform for used in the fabrication of an optical fiber which does no suffer adverse effect of Ge.sup.2+.